Security management systems are typically employed in correctional facilities, such as prisons, as well as buildings intended for other purposes where restricted access is required. Some examples of such systems include those sold under the names Pagasus, Card key and Access. In general, these systems are proprietary, and components from one system will not work with components from another system. Additionally, any modifications to the hardware or software must generally be made by the originally manufacturer.
In a typical prior art security management system (SMS) a number of field devices, perhaps several hundred or even thousands, are wired back via various circuits to a centralised SMS control unit. Typical field devices include infra-red motion detectors, read switches on doors and windows, glass breakage tapes on windows, smoke or heat detectors and tamper switches. Each of these field devices includes a switchable element which is triggered when an abnormal or specified condition occurs, for example a read switch detects that a door is opened, an infra-red motion detector senses movement or a smoke detector senses smoke in the air. The switchable element may be a normally open contact (ie., it closes when triggered) or it may be a normally closed contact (ie., it opens when triggered).
In general, a first resistive component is connected in series with the switchable element and a second resistive component, referred to herein as a field resistor, is connected in parallel with the switchable element. The field resistor is typically connected across the terminal block of the field device at the time of installation. If more than one field device is connected within a particular circuit, the switchable element of each of those devices is connected in parallel with the field resistor. In this configuration, the field resistor is usually connected across the switchable element of the last field device on a line extending from the SMS control unit.
FIG. 1 shows a typical example of a single line circuit connected to a switchable element SW1 of a single field device. The circuit includes a first resistive component R1 in series with the switchable element SW1 and a second resistive component R2 (field resistor) in parallel with the switchable element SW1. Several field devices may be connected to this circuit and, in that event, the switchable elements of those field devices would be connected in parallel with the field resistor R2. In practice, the field resistor R2 would be connected to the field device farthest from the input terminals 1, 2 of the SMS control unit.
On considering the circuit shown in FIG. 1, it will be appreciated that the line resistance measurable at input terminals 1, 2 of the SMS control unit will change when the switch SW1 closes. With the switch SW1 in the open position the line resistance will be R1 plus R2. With the switch SW1 in the closed position the line resistance will be R1 alone. The SMS control unit determines the status of the switch SW1 (opened or closed) by continuously measuring the circuit resistance of the line connected to its input terminals 1, 2.
Each manufacturer of SMS equipment specifies a particular value-of field resistor to be connected across the last field device in a line. Typical values may be 2 kΩ, 4.7 kΩ or 10 kΩ. The resistance of the cable itself is in general insignificant in comparison to the values of the resistive components R1 and R2 involved in the circuit. In many applications, the series resistor R1 is the same value as the field resistor R2. In any particular installation, wherein all lines are connected to a single SMS control unit, the field resistor R2 for each line of the system in the same value.
The various field devices in a particular installation are often supplied by other manufacturers and those devices can generally be used with any SMS control unit. This is because the field devices merely contain a switching element and the field resistor is connected during installation of the system. In some cases however, the supplier of the SMS control unit may also supply field devices and, in those cases, the field resistor may be hard wired within the device, rather than being externally wired across the terminal block at the time of installation. In that event, the field devices can only be used with the same brand of SMS control unit.
These factors cause a few problems when the owner of an SMS system needs to upgrade or modify its system. Because each line connected to the system includes a field resistor of a particular value, the owner is forced to return to the original supplier of the SMS in order to provide an upgrade. Alternatively, the system owner must rewire each of the lines connected to the system and replace the field resistor with a different value, as specified by the supplier of the new SMS control unit. Where the resistor is built into the field device it cannot be changed and the system owner is forced to also replace each of the devices if it wants to change to a different brand of SMS control unit.
Typical SMS systems include an operator interface providing a graphical representation of the system being monitored and controlled. The software employed in the interface is proprietary and cannot be changed by the user. Any modification to the operator interface thus needs to be made by the original supplier and this makes the owner vulnerable to excessive ongoing maintenance costs by the supplier.
In an attempt to remove this dependency on the original supplier, the present inventor has in the past developed a universal replacement for a proprietary SMS system using a standard programmable logic controller (PLC) and analog input cards. This provided a flexible solution which could be programmed to cater for a wide variety of field resistor values. Any PLC could be used to replace the proprietary system without having to change the field resistors, thus saving considerable installation time. The programming of the PLC is more time-consuming, because all processing is done within the central processor of the PLC and this needs to be programmed using conventional ladder logic, but overall installation time is reduced. The main problem with this approach in a commercial installation, however, is the high cost of analog input cards for commercially available PLCs. The cost of these cards makes this form of PLC-based SMS prohibitively expensive for large installations.
There therefore remains a need for a flexible system which can reproduce the function of a security management system, or similar systems, or which can be used in conjunction with standard and commonly available hardware and software to provide the necessary functionality.